Noctilucent clouds and the mesospheric water vapour: the past decade

Abstract

Abstract. The topic of this paper is the sensitivity of the brightness of noctilucent clouds (NLC) on the ambient water vapour mixing ratio f(H 2 O). Firstly, we use state-of-the-art models of NLC layer formation to predict NLC brightness changes in response to changes in the 80 km mixing ratio f(H 2 O) for the two cases of ground-based 532 nm lidar observations at 69 • N and for hemispheric satellite SBUV observations at 252 nm wavelength. In this study, we include a re-evaluation of the sensitivity of NLC brightness to changes in solar Lyman α flux. Secondly, we review observations of episodic changes in f(H 2 O) and those in NLC brightness, the former being available since 1992, the latter since 1979. To this review, we add a new series of observations of f(H 2 O), performed in the Arctic summer at the ALOMAR observatory. The episodic change exhibited by the Arctic summer means of f(H 2 O) turns out to be quite different from all those derived from annual means of f(H 2 O). The latter indicate that since 1996 a significant reduction of annually averaged upper mesospheric water vapour has occurred at low, mid, and high latitudes. These decreases of f(H 2 O) have been observed over the same time period in which a slow increase of SBUV NLC albedo has occurred. From this scenario and additional arguments we conclude that the cause for the observed long-term increase in NLC albedo remains to be identified. We close with comments on the very different character of decadal variations in NLC brightness and occurrence rate.

The High Resolution Chirp Transform Spectrometer for the Sofia-Great Instrument

“…This high absorption shows the equilibrium temperature to be much lower than the continuum temperature at the upper region of the atmosphere, where the broadening of the line is minimum (see weighting functions in Lellouch et al (1991)). This means lower temperatures in the middle atmosphere for the equatorial region and a colder lower atmosphere for the winter hemisphere, which is consistent with our global simulations as obtained with the MAOAM-GCM (Martian Atmosphere Observation And Modeling -General Circulation Model) (see Villanueva, 2004).…”

“…Furthermore, upon the integration of a CTS spectrometer into the Heinrich Hertz Submillimeter Telescope (HHSMT) at Mt. Graham, it has been applied to address a wide range of topics of modern astrophysics (Villanueva, 2004;Hofstadter et al, 1999), from questions about comets, planetary atmospheres and the interstellar medium in the galaxy, to investigations related to the early Universe. Recently, a CTS provided high quality spectra of comet 2002 T7 (LINEAR) and the Earth from the Microwave Instrument for the Rosetta Orbiter (MIRO), the first deep space mission carrying a submillimeter heterodyne instrument (Beaudin et al, 1998).…”

“…A more specific description of the transform performed in the newly developed instrument can be found in Villanueva (2004). The dispersive elements are SAW (Surface Acoustic Wave) filters, which have as impulse response a chirp signal or a linear frequency modulated signal.…”

“…Since the early nineties continuous ground-based measurements of water vapor (22/183.6 GHz) and ozone (142 GHz) have been performed. This unique database was crucial in our understanding of the annual and inter-annual variation of the Earth middle atmospheric water vapor (Hartogh, 2000;Seele and Hartogh, 1999), the building mechanism of noctilucent clouds (NLCs) (von Zahn et al, 2004), the relationship between brightness and frequency of NLCs, the advection of air during a stratospheric warming (Seele and Hartogh, 2000) and the middle mesosphere maximum of ozone . Furthermore, upon the integration of a CTS spectrometer into the Heinrich Hertz Submillimeter Telescope (HHSMT) at Mt.…”

Polar mesosphere summer echo strength in relation to solar variability and geomagnetic activity during 1997–2009

“…An increase in solar Lyman α radiation leads to increased water vapour photo-dissociation (Brasseur and Solomon, 1986). This should result in less ice-particle formation (von Zahn et al, 2004) and hence should affect PMSE. Variation of solar radiation could also influence atmospheric temperature via production of ozone after photodissociation of O 2 (Robert et al, 2010) and via changing atmospheric dynamics.…”